Sliding Dovetails - Canadian Woodworking Magazine

Router Skills: Of the many styles of dovetail joints, none are as useful or strong as the sliding dovetail.

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Sliding Dovetails



Photos by Rob Brown

When building this simple shelf for my new-born daughter Annabelle’s bedroom, I wasn’t aim­ing for high style, but just because the design isn’t strong, this doesn’t mean that the shelf shouldn’t be.

The sliding dovetail joint has a number of advantages: it’s mechanically strong, provides a decent amount of glue sur­face, is reasonably easy to machine and is attractive when assembled. You don’t need a whole lot of fancy tools to cre­ate it, just a router, router table and a dovetail bit. However, in spite of all the benefits, woodworkers tend to shy away from the sliding dovetail because of the perception that it’s a difficult joint to machine. This is a shame. With a cou­ple of simple steps and a measure of patience when it comes to fine tuning the thickness of the tenon, it’s probably the easiest of the dovetail joints to machine. Keep that thought in mind: patience. That’s the key to the whole process.

When machining this joint, it’s a good idea to make a few extra pieces. They can be used to assist with setup, as well as acting as backups if your patience leaves you suddenly and you machine the tenon too small. Be sure the parts that are meant to receive a tenon are all exactly the same thickness. That said, consistent thickness of the dovetail groove doesn’t matter as much.
 
Choosing the Dovetail Bit
There are a few guidelines when choos­ing a dovetail bit. Generally speaking, the larger the bit, the stronger the joint. However, don’t use a bit that is wider at its tip than the thickness of the part that will have the dovetail tenon on its end. Use a router bit with a ½" shank as this will reduce the chances of the bit break­ing while you’re plowing the groove. Before you begin, make sure the surface of your router table is dead flat and your router base plate is flush with the router table’s surface. Any slight variation will make the process more difficult.

Install the bit into your router and adjust the height of the bit in your router table. This, admittedly, is bit of a bal­ancing act. The higher the bit, the more mechanical strength and glue surface area the joint will have. But keep in mind that if you machine the dovetail groove across the work piece you will be weak­ening it because you are removing some of its thickness. Try to keep the depth of the dovetail groove between ⅓ and ½ the thickness of the part. For example, if the gable of a chest of drawers is ¾" thick, machine the dovetail groove between ¼" and ½" deep. Any deeper and you will weaken the gable too much.
 
Do the Easy Half First – Dovetail Grooves
The first of the two parts to machine is the dovetail groove. It’s much easier to fine tune the thickness of the tenon to fit the groove than it is to fine tune the thickness of the groove to fit the tenon. With the height of the bit set, position the fence to the proper distance from the bit. You could push the part to be grooved over the bit right now, but if you’re using a larger dovetail bit, the cut may be too heavy to be safe and smooth. Instead, push the work piece over the bit, making a small dovetail notch in the edge of the work piece (usually about ½" into it). Take the work piece to the table saw and, with a number of passes, remove the majority of the waste, being careful to use the dovetail notch you just made as a guide. This will lighten the load on the router when you machine the dovetail groove. Repeat this with all work pieces. Return to the router table and machine all of the dovetail grooves required, being sure to keep each part in contact with the router table’s surface as you make the passes. Remember, the work piece must travel from right to left, ensuring the rotation of the bit is pull­ing it into the fence as opposed to away from it.
 

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Ease the work for the router bit – Hog out the middle of the dovetail groove at the table saw to make the router cut easier.

The Tricky Half – The Dovetail Tenons
With the easy half compete, you may now focus your attention on the dove­tail tenons. The fit should be such that the joint fits together with a medium amount of hand pressure. You want a clean fit. It’s very easy to make the dovetail tenon too thin, leaving the joint loose, so this is where your patience is essential. Even a tenon 1/32" too thin will produce a weak joint. The key is to sneak up on the final ‘perfect’ fit. Slow and steady wins the race.

Keep the router bit set at the same height to make the dovetail tenons. The work pieces that will have tenons on their ends will run vertically, mean­ing you will need a fence with a high enough face to support the work pieces. At least 5" high is recommended, but higher is even better. You might also need a 90° push block to keep narrower pieces perpendicular to the table’s sur­face. Position the bit so that only a small portion (⅛" or so) of it is projecting beyond the plane of the fence. When you pass both sides of the work pieces past the bit, the surfaces will be scored slightly, reducing the chance of chip­ping. After each work piece has been scored on both surfaces, move the fence away from the bit slightly. Now keep one thing in mind: if you move the fence 1/16" away from the bit you will be removing ⅛" from the dovetail tenons’ thickness. This is because you are removing 1/16" from both sides of the tenon. After you move the fence it’s a good idea to use a test piece to see if you have gone too far. Machine each piece on both sides, sneaking up on the final perfect fit.
 
Patience Saves Time and Material
There will be a time near the end where you will almost have the fit you’re looking for. This is where patience comes into play. Move the fence away from the bit ever so slightly. At this stage, make a pass on one side of the work piece only, then check if you have the perfect fit. If it’s still a bit snug, turn the work piece around and machine the other side. You can number the faces ‘1’ and ‘2’ to keep track of the sides you have already machined. At this stage you’re removing about 1/128" per pass, so if after the first pass you achieve that perfect fit, a tenon slightly offset will not be noticed. Machine all of the dovetail tenons to their final thickness. It’s a good idea to check that each one fits its mate perfectly. Sometimes, even with all these precautions, you end up with slightly different thicknesses and you have to adjust the cut ever so slightly for the odd tenon.

With a narrower work piece (a 3" wide rail) the fit needs to be perfect throughout its width. With a wide work piece (the 18" deep writing surface in a secretary desk), a perfect fit will be very difficult to assemble once you introduce glue to the wood. In this case there is something you can do to make assembly a little easier. Once you make the final pass on the dovetail tenon and the perfect fit has been achieved, move the fence about 1/64" away from the bit. Now remove a bit more material from the center 50% of the dovetail tenon, leaving either end with the perfect fit. This will reduce some of the friction near the center of the tenon, make assembly easier, and though 1/64" less, the dovetail tenon will still be housed in the dovetail groove well enough to provide strength. Either way, a clamp or mallet with striking block will be handy to have during glue-up. Once you add glue to the mix, you’re working against the clock. This is a way of stacking the odds in your favour.

With a bit of practice you will be able to set up and machine this joint quite efficiently. There will even come a time where you’ll think you don’t need to sneak up on the perfect fit. This will also be the day you have to remake some parts because you cut the tenon too thin. Trust me – I’ve done it more than I like to admit.
 

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Patience please – Start with a scoring cut on either side then sneak up on the tenon size to ensure a good fit.
 

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What is the perfect fit – The two parts of the joint should fit together so that there are no gaps but still able to slide.
 

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End result – What you are aiming for is a gap-free fit that all but disappears.
 
Feather Boards
In order to ensure equal and constant pressure on the work piece, use feather boards, which can be easily installed on most router tables and fences made today. If you have made your own router table and fence, cut some grooves to accept these handy jigs. All it takes is a bit of bow in the board into which you are putting a dovetail groove to mess up the whole operation. A feather board will give you some extra assurance.
 

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